1. Field of the Invention
This invention relates to servo address mark detection in a disk drive, and more particularly, relates to a circuit for automatically detecting a servo address mark (SAM) and compensating for servo timing delay in response to misdetection of a servo address mark in a disk drive.
2. Related Art
Hard disk drives typically include one or more magnetic disks defining a multiplicity of concentric data tracks. Magnetic disk drives having high data density generally rely upon head position control systems for moving a selected transducer (head) from a departure track to a destination track location during track seeking operations, for settling the head at the vicinity of the destination track during track settling operations, and for following the centerline of the destination track during track following operations when data information is written on or read from the disk. Contemporary disk drive architectures having head position control systems using servo information detection are disclosed in U.S. Pat. No. 4,823,212 for Sampled Servo Code Format An System For A Disc Drive issued to Knowles et al., U.S. Pat. No. 4,977,472 for Servo Address System issued to Volz et al., U.S. Pat. No. 5,210,660 for Sectored Servo Independent Of Data Architecture issued to Hetzler, and U.S. Pat. No. 5,515,371 for Track Sector Detection System For Rotating Disk Data Storage Apparatus issued to Takei.
Generally, disk drive designs embed servo head position information within the concentric data tracks of a magnetic disk. This embedded information is usually recorded in evenly spaced apart areas or sectors of the track. The embedded servo information includes servo head position and track/data identification fields, and typically includes a unique servo address mark (SAM) pattern which is provided to resynchronize timers for recovering the servo head position and the track/data identification field information, and which mark in time expected arrival of the next embedded servo sector. As a result, reliable detection of the servo address mark pattern in each servo sector is essential for precisely marking in time not only the servo sector and its positioning information fields, but also the following user data storage area. If a servo address mark (SAM) is corrupted or misdetected, subsequent servo sectors may be overwritten. Recent efforts to ensure reliable detection of servo address mark pattern in a disk drive are disclosed, for example, in U.S. Pat. No. 5,231,545 for Fault Tolerant RLL Data Sector Address Mark Decoder issued to Gold, U.S. Pat. No. 5,384,671 for PRML Sampled Data Channel Synchronous Servo Detector issued to Fisher, U.S. Pat. No. 5,420,730 for Servo Data Recovery Circuit For Disk Drive Having Digital Embedded Sector Servo issued to Moon et al., U.S. Pat. No. 5,442,499 for Method Of Detecting Sector Servo Information In A Disk System issued to Emori, U.S. Pat. No. 5,477,103 for Sequence, Timing And Synchronization Technique For Servo System Controller Of A Computer Disk Mass Storage Device issued to Romano et al., and U.S. Pat. No. 5,544,135 for Fault Tolerant Servo Address Mark For Disk Drive issued to Akin, Jr. et al. While the servo data detection techniques of Moon '730, Emori '499 and Akin '135 require sophisticated circuitry and possess their own merits, I have noted however, that further improvements in the servo address mark detection can be made to reliably detect the servo address mark (SAM) of a pertinent servo sector where the servo address mark (SAM of the servo sector is not detected in order to prevent recorded servo sector information from being destroyed during subsequent write operations.